668 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



or sand; if the amount of alkaline carbonate suffices, the magnesium salt may 

 be added to the deposit in large amounts. 



Whatever may be the exact chemistry of the process (perhaps mass-action in 

 control) dolomitization of recently deposited calcareous matter does take place 

 beneath the sea floor. The detailed studies of the boring at the Funafuti atoll have 

 proved that fact beyond peradventure.* From the accounts of the different 

 writers of the Funafuti report one must conclude that the extensive dolomitiza- 

 tion in this case has been facilitated by the porous nature of the coral growth 

 and coral-shell debris; the porosity permitting of the circulation of sea water. 

 The impalpable, inorganic calcareous muds of the pre-Cambrian would evidently 

 not be porous. It seems, therefore, probable that the dolomitization took place 

 at the surface of the pre-Cambrian calcareous mud, the conditions there favour- 

 ing the formation of the double salt at or very near the contact of mud and 

 main water body. To-day the same chemical conditions seem to be found in the 

 oceanic areas only at some depth below the surface of coral reef or of calcareous 

 sands of direct organic origin. 



There are yet other ways in which magnesium carbonate may be elaborated 

 from sea-water — through certain algas and a few animals known to secrete 

 magnesium carbonate along with the dominant calcium carbonate of their hard 

 structures, or, finally, through the local evaporation of sea water. However, 

 the cmantitative value of all these sources just mentioned may well be suspected 

 to be but subsidiary to a more general cause of dolomite formation. Most of 

 the world's magnesian limestones and dolomites seem to owe their origin neither 

 to the secretions of special organisms nor to evaporation. The special organisms 

 are too rare in one case; evaporation must be too local for the other case. 



The scope of the present report does not permit of a critical discussion of 

 the many published theories concerning the dolomites. It may only be stated 

 that, if we accept the leaching hypothesis or the hypothesis that dolomite is the 

 result of metamorphic processes by which magnesium comes to replace calcium 

 in ordinary limestone, we meet with very grave difficulties, long ago stated 

 and never overcome. The rapid alternation of clean-cut beds of pure or nearly 

 pure calcium carbonate with other clean-cut beds of magnesian limestone or 

 dolomite is a fact hardly to be reconciled with these metamorphic theories. The 

 metamorphism is, by these theories, accomplished through the activities of cir- 

 culating underground waters; yet it seems impossible that such wholesale meta- 

 morphism could leave the original bedding so well marked. The alternation of 

 clean-cut beds as described is a prominent fact illustrated, for example, in the 

 pre-Cambrian formations of Montana and British Columbia. The facts of the 

 field speak rather for an original deposition of the two carbonates arranged in 

 very nearly their present relations. 



It is scarcely necessary to dwell on the effect of burial on the chemical pre- 

 cipitate of basic magnesium carbonate. Pressure and a heightened temperature 

 have gradually driven out the water of crystallization. The simultaneous forma- 

 tion of the double carbonate, dolomite, might be expected where both carbonates 



* The Atoll of Funafuti. Published by the Royal Society of London, 1!K)4. 



